Alternative Functions
by Homicidal Whispers
Summary: In which a couple of boys touch dicks and do math.


Armin gets a text in the middle of class and he smiles; even without looking, he already knows who it's from. Moving innocuously to avoid arousing his teacher's suspicion, he sets his phone inside his desk and checks the message.

_hey_, it reads.

_How many times do I have to tell you not to text me in class?_

There's a bit of a lag before the phone vibrates again. _you know it all already so it doesnt matter_.

_That doesn't make it okay._

_whatever_. Barely a second passes before he's got another. _im off today btw_.

Armin realizes he's smiling stupidly at his desk and schools his face into a neutral expression before the teacher can notice and get suspicious. _Yes, I know. What about it?_

_idk you could come over if you wanted or something._

Armin puts the phone down, deliberately leaving Eren hanging. It's kind of mean, he knows, but he likes it when Eren panics. He hasn't yet realized that Armin's committed to this for life.

For a few minutes, Armin concentrates on doing his work, partly because he wants to irritate Eren and partly because the teacher really has been eyeballing him warily. Class is about to end, so he finishes up his work before sending one last message before the bell rings.

_I'll be there_.

_great. ill pick you up after school, usual place._

-/-

Armin waits by his locker after school for Eren to show up. Even if he's working that day, Eren always makes it a point to see Armin before he leaves; sometimes he'll even be able to give Armin a ride home. It's a moot point though, because Eren's not working today. It's a Friday and Armin's already told his grandfather that he'll be sleeping over by Eren. All that's left is for them to go home.

"Armin," Eren says as he arrives, breathless from rushing. He pulls Armin into his chest, winding his arms tightly around him and pressing a quick kiss to his lips. Earlier on, when their relationship was newer, this type of things would garner stares. Now though, over a year later, they don't even get a glance; everyone is used to it.

"Are we waiting for Mikasa?" he asks.

"Not today. She's got a match in the morning, so she's staying at Annie's tonight," Eren answers.

Armin frowns. "Isn't Annie her biggest competition?"

"Nah. Annie's not wrestling tomorrow, so she's helping Mikasa prepare." Eren shrugs. "I don't know. It doesn't make any sense to me."

"Annie probably just wants to make sure Mikasa gets to finals so they can fight each other."

"Girls are weird," Eren says. "They're so roundabout. Isn't it easier to just say what you mean?"As he speaks, he casually takes Armin's bag and swings it over his back.

"I can carry it, you know," Armin points out. It's an age-old argument of theirs. Eren's been doing it since they were young kids. As always, Eren flashes his teeth in a grin.

"I know you can do it," he answers. "But I want to do it for you. Besides, it's not like I have anything to carry myself." That is true enough. Eren somehow manages to get through the entire day with just one notebook and maybe a pen. Armin, on the other hand, is left with a giant knapsack stuffed to the brim with books and binders. To be honest, it's so heavy that it's a relief when Eren takes it; that's why he never argues the topic too hard.

As always, their departure draws attention. Armin blames it on Eren's ostentatious bike – it's loud and unapologetic, just like he is. Armin takes the helmet that Eren hands back to him and clings onto the back of Eren's jacket as they peel out of the student parking lot. They're going too fast because Eren's a show off, but Armin can feel the rumble of Eren's laughter under the roar of the engine. It's infectious. Armin can feel himself smiling too. When Eren whoops with adrenaline, Armin joins him.

It doesn't take too long for them to get back to the apartment that Eren and Mikasa share, not at the speed they were going. "Get the door for me?" Eren asks. "I'll go lock up the bike."

"Sure," Armin agrees. He fumbles in his bag for his key ring and opens the door. He leaves it open after he enters, waiting for Eren to come back from the parking garage. In the mean time, he makes himself comfortable – kicking off his shoes, throwing down his sweater, and wandering.

There are certain downsides to dating an emancipated minor.

The first is how little free time Eren has. In order to afford the apartment, both of them have to work. Between school and his job, Armin never gets time to see Eren as much as he wishes he could. The weekends, some of the best times for a normal student, are usually the days that he is the busiest.

There's the fact that Armin sometimes feels like he's more of a parent to Eren than a boyfriend. With no parental advisory, Eren, like the typical teenager, tends to go a little bit out of control. Armin constantly harasses him, nagging the way a mother would. _When was the last time you did laundry, Eren? Did you remember to finish your homework? Stop eating so much junk food!_ He feels self-conscious about it sometimes, but he knows Eren secretly likes it. He'll laugh, reach up and pull Armin down into his lap and say, "Stop nagging." Despite that, he'll go do the laundry, go finish his homework, and cook dinner for the night.

There's the inevitable depression. Eren likes to pretend that he's not bothered by how he came to be living on his own in the first place, but Armin's known Eren forever. He knows what happened, and he knows how Eren feels about it. Eren's mom had been murdered some years ago. She had been the centerpiece of their family, and without her, they all fell apart. His dad had gotten more and more withdrawn, until Eren and Mikasa had been forced to go to court. They haven't seen or heard from him sense. Armin catches Eren crying sometimes, and he never says anything about it. He just pulls him into his arms and rides it out with him.

There are plenty of downsides, but there are easily just as many advantages, the most obvious of which is that Eren always has a free house; that Mikasa doesn't care about any noise that they might make only yet another bonus.

Armin hears Eren come crashing into the house, hears the slam of the door behind him and a serious of thumps that's probably him bumping into a wall or dropping something. "Armin?" he calls.

"In the living room," he yells back, and he can hear Eren heading for him. Moments later, he appears in the doorway, smilinh sheepishly.

"You already called your grandpa, right?" Eren asks him, dropping into the couch beside him.

"Yeah," Armin confirms. "He's not expecting me back until tomorrow morning at the earliest." It's one of the benefits of having been friends for so long; Armin's grandfather never questions him staying over by Eren.

"Great," he says. "What do you want to do now? We can order some takeout, stream a movie."

"No takeout. You're cooking later," he says firmly. "But a movie sounds good."

They can't decide on a movie that they haven't already seen dozens of times, so they eventually pick one at random from their recommendations on Netflix. It turns out to be some sort of romantic comedy – a genre to which Eren will never admit to being secretly in love with – with a healthy topping of action, more to Armin's taste. It's nothing special though, and as the time passes and the bowl of popcorn dwindles, both of their attentions begin to dwindle.

Over the course of the movie, they've shifted, going from upright to mostly horizontal on the couch. They're pressed together, Armin's back against Eren's chest so they can both see the screen. Somewhere around three quarters of the way through, Eren's hands start to wander. They find their way from around Armin's waist to his hips and legs, then up Armin's shirt and inside his pants.

Armin notices it, of course, and he notices the evidence of Eren's growing interest. Still, he remains still and quiet, content to let himself be touched but neither giving nor taking anything.

The movie ends and Eren pauses the credits. He looks at Armin hopefully. "Now what?" he asks. The answer he wants to hear is written all over his face.

Armin sits up and stretches. Eren stares at the small patch of skin between his shirt and pants that is revealed through the movement. "Now," Armin says sweetly, "we study."

It takes a moment for that to process. "What?" Eren asks.

"Studying," Armin repeats. "You've got a trig test on Monday, don't you? Go get your textbook. I'll help you."

Eren's considering arguing, that much is clear. Still, he really does need the help. Math has never been his strength, but he's never done this poorly before. They both know that if he doesn't do well on this test, he'll fail for the marking period.

"Fine," he says, shuffling away to his room to retrieve it.

"I'll be in the kitchen," Armin calls back.

It takes a while for him to come back. When he does, Eren plops into the chair opposite the one Armin has claimed and drops the book on the table. It's dusty, like it's never been opened. Armin looks at it and then at him pointedly. Eren just shrugs and flips to the right page.

"We're supposed to be doing the trigonometric identities or something like that," Eren says. "I really don't get it."

Armin takes a moment to look over the practice problems in the books and nods. "I remember doing these. Questions like these are really just manipulating the identities. As long as you know them, you should be alright."

"But I don't know them," Eren says.

"Well then," Armin says and stands. Easily, he pulls off his shirt and drops it to the side, fingering his hair back into place. Next his pants go off, followed by his underwear. Eren's staring at him, eyes wide. I'll just have to help you learn."

He perches himself on the table. "We'll start with the basics. The three basic trig values are sine, cosine, and tangent, which all come from values on a right triangle. Which is which?"

"Uhm," Eren says. Armin waits patiently for him to gather his scrambled thoughts. "Sine is opposite over hypotenuse. Cosine is adjacent over hypotenuse. Tangent is adjacent over opposite."

"Not quite."

"Sorry, I mean tangent is opposite over adjacent."

"Good," Armin says encouragingly. "What about the reciprocal identities?"

"Secant is the reciprocal of cosine, hypotenuse over adjacent. Cosecant is the reciprocal of sine, hypotenuse over opposite. And cotangent is adjacent over opposite."

"Perfect. This is where it starts to get difficult, I'm afraid," Armin says. He slips off of the table and crawls underneath, settling in between Eren's legs. There's just enough of a gap between the table and the chair that he can see Eren staring down at him. "There's a set of equations in the book called Pythagorean Identities. There's a way to figure them out by yourself, but for now I just want you to memorize them. Do you see them?"

"Yes," Eren answers, voice strangled. "Memorize them?"

"That's right," Armin says. He unzips Eren's pants and pulls out his dick. Eren's already mostly hard. "Memorize them."

It is with great pleasure that he goes to work on Eren's. There's no way that Eren didn't know it was coming, but he still reacts beautifully, jerking in surprise when Armin brushes his cock with his tongue and cursing when his knees hit the underside of the table.

"Alright up there?" Armin asks.

"Fine," Eren answers back, voice strained. "Pythagorean Identities. Got it."

Armin unabashedly loves sucking dick. He likes the subtle taste of it, the weight of it on his tongue. He likes wrapping his lips around the head and sucking. He likes the tinge of discomfort, suppressing the urge to gag, the sore jaw that comes with sucking for too long. He likes the feel of it sliding down the back of his throat. So without further ado, Armin opens up and guides Eren's dick into his mouth.

However, the goal is to prolong this, so he doesn't do too much for now. He works at the head of his cock, slowly sucking the tip into his mouth, letting it drag over his tongue before releasing it with an obscene noise. Above him, Eren shudders slightly and curses. He can hear the slight rasp of pages turning as he tries to focus.

"Do you think you have it?" he asks. "Shut the book so you can't cheat." Armin, beneath the table can't see whether or not Eren actually does, but he hears a slight thud above him. Eren shifts the chair back and looks down at the blonde in between his legs. Carefully, like he's not sure he's allowed, he lets his hands drift into Armin's hair.

"I think so," he says. "Sine squared x plus cosine squared x equals 1."

"Very good," Armin praises. He wraps a loose fist around Eren's dick and strokes him; this is a reward for getting it right, and he makes sure Eren can't' interpret it any other way. "What's 1 minus secant squared x?"

It's not the exact way it's written in the textbook, so there's a pause before Eren answers him. In the silence, Armin sinks down onto his dick, taking him deep in his mouth and hollowing out his cheeks and sucking back up. "Negative tangent squared x," he gasps out finally

"And what minus cotangent squared x equals 1?"

"Cosecant squared x." The answer comes much faster this time. Either these formulae are beginning to make sense to Armin's form of motivating him is working. Either way, Armin is satisfied.

"Exactly. Now find the double angle formulas."

"Yeah, okay," Eren mumbles. With one hand, Armin can hear him fumbling for the textbook again. The other, still tangled into Armin's hair unceremoniously shoves him back down. Armin goes willingly, taking Eren's cock into his mouth as deep as it will go. He swallows around him and pulls back up to the head, licking up the pearly fluid that's accumulated in the slit. He laves his tongue against the underside, following the thick vein back to the root and then sucks kisses in a neat line back to the top.

"Don't come," he says pleasantly, rubbing his cheek along the length of it. "What's sine 2x?"

"Shit," Eren says. "No, I got it. It's 2sinexcosinex."

Despite the answer being correct, he doesn't touch him again except to clamp down on the base. He can't come too quickly. "Cosine 2x?" Armin asks.

"Cosine squared x minus sine squared x, and the last one is 2tangentx divided by 1 plus tangent squared x."

Armin clamps down harder. "Try that last one again."

"2tangentx divided by 1 minus tangent squared x," Eren corrects desperately, fairly babbling. He sounds desperate, his fingers scrabbling for purchase on the wood, on the chair, on the table. His hips stutter, trying to rock into Armin's palm, but his grip is too tight for that

"Excellent," Armin says. "Now, I need you to be very good. I'm going to come up there, but I need you to focus on the book. Learn the sum formulas now. Do _not _touch yourself. Can you do that?"

Eren nods shakily. "Yes, anything," he says. "Armin _please_."

Armin ignores him. He crawls out from under the table and rises to his feet. He shakes for a moment and frowns – he hadn't realized how much his knees ached until he was off of them. The tiled floor of the kitchen had done him no favors. Eren's not watching him, his head obediently turned towards the math book.

He goes into Eren's bedroom and grabs the bottle of lube from where he knows it will be. While he's there, he settles on Eren's bed and spreads his legs. He has to prep himself, because he knows Eren will be nowhere near cognizant enough to do it properly himself. He doesn't do too much, only just enough to take the edge off. He'll let Eren finish the job later.

It's different when Eren does it. It feels better, more raw, more intense. Eren will press his fingers into him slow and intent, watching his face to see how much he likes it. He'll whisper all he plans to do with Armin into his ear as his fingers fuck in and out of him. He curls his fingers in just the right way and make him feel so alive.

Eren likes to prolong it too. He'll patiently fuck Armin open with his fingers until Armin screams and then he'll keep going. He'll watch Armin shake until he begs to be fucked properly. "I could get off just from watching you fuck yourself on my hand," Eren had told him once.

Doing it himself isn't too bad, but in comparison to when Eren does it, it's downright uncomfortable. He has to contort his body to get his hand where it needs to be. He can never get his own fingers as deep as he wants them and soon, his wrist and joints start to ache.

He returns to the kitchen to find Eren hunched over the book, his hands gripping the table to prevent him from gripping his own cock. "Eren?" he asks and Eren turns to face him, eyes wild and dark.

"Yeah?" he says tensely.

"Here's what's going to happen," Armin tells him. "You're going to get on that table and I'm going to ride you. While you fuck me, I'll ask questions. If you get any wrong, I'll stop and I'll tie your hands to the door for the rest of the night so you can't finish yourself off. Sounds good?"

He makes a sound that Armin can't classify, something between a choked whine and a whimper. Still, he doesn't try to argue, just nods. "Yes," he says. "Whatever you say, Armin, _god_."

"Just Armin's fine," he says mildly. "Table."

"Should I take these off first?" Eren asks, gesturing to his clothes. There had been a time when Armin was still insecure – about his body, about his relationship. There was never a doubt that Eren was the more fit one out of the two. There had been a time when he was too afraid to get undressed with Eren in the room; when he insisted that the light remain off; when he couldn't get naked unless Eren was too. Now though, he's had plenty of time to understand that Eren loves everything about him, including the way he looks.

"No," he says. "Stay like that."

Eren gets on top of the table and lies back while Armin climbs astride his lap, one knee on either side of Eren's body. He reaches back to grasp Eren's dick. With the way Armin's been teasing him all night, he's obscenely hard, full and red.

"Sine x times cosine y minus cosine x times sine y equals?"

"Sine x plus y," Eren answers breathlessly.

"Good," he replies and kisses him lightly before he sinks down.

Eren groans and his hands come up to clutch at Armin's biceps. "What do you want me to do?"

"I want you to be a good boy and fuck me like you mean it," he says. He plants his arms on Eren's chest and uses it as leverage to pull himself back up before dropping. Eren thrusts, lifting his hips up to meet Armin on his way back down. The kitchen echoes with the slap of skin on skin.

"Tangent of x plus y," Armin murmurs.

"I don't know, I don't know, just keep – _Armin_," Eren babbles.

"You don't know?" Armin says slowly. "Should I stop, then?"

"It's tangent x plus tangent y over one plus tangent x times tangent y," he says in a stream. "Don't stop, don't _stop_."

Armin knows there's no way Eren will last, not when he's prolonging this for so long. Already, he's starting to get messy and uncoordinated, falling out of the rhythm that Armin established. He takes himself in his hand, trying to jack himself off and fuck himself on Eren's cock at the same time. He's close, too.

Too soon, Eren clenches up and comes with a muffled cry of Armin's name, filling Armin's ass. His hand comes around to Armin's front, helping to jack him off. It doesn't take too long before Armin follows him over the edge.

For a moment they remain on the table, huffing to catch their breaths, covered in sweat and semen. They don't move until getting stuck together becomes a real concern, at which point they head for the shower. After, Armin steals some of Eren's clothes and they head to the kitchen to start dinner.

"I think you'll be ready for the test," Armin says. "I'll help you work on it it some more tomorrow."

Eren looks confused stuck somewhere between anticipation and dread and not sure on which side of the spectrum he should lie.

-/-

Armin's plan works; Eren passes that test with flying colors. His teacher is so confused that she accuses him of cheating and refuses to believe him until he demonstrates that he actually does know the material, and he knows it well.

Armin thinks part of the reason the teacher mistrusts him is the look on Eren's face while he tries to explain his sudden transformation – he can't even think of trig anymore without going red.

-/-

**AN: **an awesome person was all "i love bossy armin" and i was all "i can do that."

if you think i didn't pull out my old trig notebook for this than you're wrong


End file.
